1. Field of the Invention
The present invention relates to a lighting circuit of a cold cathode discharge lamp, and more specifically, to a circuit for adjusting luminance of the cold cathode discharge lamp by a duty light adjusting system.
2. Description of the Related Art
FIG. 5 is a schematic constructional view showing one mode of a lighting circuit of a cold cathode discharge lamp in which luminance is controlled by a duty light adjusting system.
As shown in this figure, a high frequency voltage (about 60 kHz) generated by a ROYER oscillating circuit 12 is increased by a transformer 13 and the cold cathode discharge lamp 11 is lighted by this increased high voltage (from 600 to 1600 V). In the lighting circuit of the cold cathode discharge lamp having such a circuit construction, an oscillating operation of the ROYER oscillating circuit is normally turned on/off every constant period and a ratio of a turning-on period to the constant period, i.e., a duty ratio of the oscillating operation of the ROYER oscillating circuit is changed by a PWM (Pulse Width Modulation) circuit 17 so that luminance of the cold cathode discharge lamp is adjusted. In the mode shown in FIG. 5, a luminance adjusting waveform 16 of H/L (High state/Low state) outputted from an IC (comparator) X4 of the PWM circuit 17 is inputted to a switching circuit 14 and the operation of the ROYER oscillating circuit 12 is controlled by an output signal from this switching circuit 14. The duty ratio of the luminance adjusting waveform 16 of the cold cathode discharge lamp 11 is controlled by the magnitude of a light adjusting signal voltage inputted to an inversion input terminal of the comparator X4.
FIG. 6 shows a voltage waveform inputted to an input terminal of the comparator X4 and a voltage waveform outputted from an output terminal of this comparator.
A triangular wave voltage Va is inputted to a non-inversion input terminal of the comparator X4 and light adjusting signal voltages 18 (Vb1, Vb2, Vb3) are inputted to the inversion input terminal of the comparator X4. Rectangular wave voltages Vo1, Vo2, Vo3 corresponding to these light adjusting signal voltages Vb1, Vb2, Vb3 are respectively outputted from the output terminal of the comparator. For example, when the light adjusting signal voltage Vb1 is inputted to the inversion input terminal, this light adjusting signal voltage Vb1 and the triangular wave voltage Va inputted to the non-inversion input terminal are compared with each other. A voltage level of the output terminal is set to a high (H) state in a period in which the triangular wave voltage Va is higher than the light adjusting signal voltage Vb1. In contrast to this, the voltage level of the output terminal is set to a low (L) state in a period in which the triangular wave voltage Va is lower than the light adjusting signal voltage Vb1. Namely, the rectangular wave voltage Vo1 is outputted from the output terminal. Similarly, when light adjusting signal voltages Vb2, Vb3 are inputted to the inversion input terminal, light adjusting signal voltages Vb2, Vb3 are respectively compared with the triangular wave voltage Va, and rectangular wave voltages Vo2, Vo3 are respectively outputted from the output terminal.
Thus, a PWM signal varying a ratio of the high (H) period to the low (L) period in one cycle depending on the magnitude of a light adjusting signal voltage is outputted, and the high (H) period is lengthened as the light adjusting signal voltage is reduced. This output voltage is inputted to a transistor Q1 of the switching circuit 14, turns on transistors Q1 and Q2 during the high (H) period and makes the ROYER oscillating circuit 12 start oscillating operation, so that a high frequency voltage is increased by the transformer 13 and is applied to the cold cathode discharge lamp 11. Luminance of the cold cathode discharge lamp 11 is increased as the light adjusting signal voltage is reduced, i.e., as a duty ratio of output waveforms is increased. The duty ratio of the PWM signal is normally set such that this duty ratio varies in a range from 10 to 100% when the light adjusting signal voltage varies from 0 to 5 V.
FIG. 7 is a constructional view showing a conventional example of the lighting circuit of the cold cathode discharge lamp using the duty light adjusting system.
A ROYER oscillating circuit 12 is a voltage resonance type circuit constructed by transistors Q3, Q4, a capacitor C2 and a transformer (T1) 13. As mentioned above, when the transistors Q1, Q2 of the switching circuit 14 are turned on, a direct current bias is applied to the transistors Q3, Q4 of the ROYER oscillating circuit 12 from a DC power source (12 V) through the transistor Q2 and a resistor R8 so that the ROYER oscillating circuit 12 is oscillated. In this example, an oscillating frequency of the ROYER oscillating circuit 12 is set to 60 kHz and a secondary voltage of the transformer 13 is increased such that an alternating voltage from about 600 to 1600 V.sub.P-P is generated on a secondary side of the transformer 13.
The triangular wave voltage Va inputted to the non-inversion input terminal of the comparator X4 that constitutes the PWM circuit 17 is generated by operational amplifiers X2, X3. A rectangular wave voltage is first generated by positively feeding an output voltage of the operational amplifier X2 back to a non-inversion input terminal through a resistor R14. Zener diodes ZD2, ZD3 between the output terminal and an inversion input terminal of the operational amplifier X2 are connected to set a wave height value of the rectangular wave voltage to a constant value. The rectangular wave voltage, that is, the output voltage of the operational amplifier X2 is inputted to an inversion input terminal of the operational amplifier X3. The operational amplifier X3 forms an integrator and is fed back from an output terminal to an inversion input terminal through a capacitor C6. Thus, the inputted rectangular wave voltage is integrated and is outputted from the output terminal of the operational amplifier X3 as a triangular wave voltage of the same frequency as the rectangular wave voltage. A frequency of the triangular wave voltage is normally set to from 100 to 600 Hz. A three-terminal regulator X1 is used as a power source for supplying a power voltage to the above operational amplifiers X2, X3 and the comparator X4. The power source voltage can be stably supplied irrespective of a change in the power source voltage (12 V) by using the three-terminal regulator X1, so that a change in the PWM signal voltage can be reduced.
However, due to its limited performance, the power source voltage would vary by about .+-.10%. Accordingly, when the power source voltage (12 V) varies by .+-.10% in the above conventional example, a primary voltage of the transformer 13 is also varied by .+-.10%. As a result, the increased secondary voltage of the transformer 13 is varied (by .+-.10%), so that an electric current flowing through the cold cathode discharge lamp 11 is also changed and so is the luminance.
Therefore, a circuit of the following system has been used to prevent this change in luminance.
FIG. 8 is a view showing the construction of a circuit using a DC/DC converter 20 to reduce the change in luminance caused by the change in the power source voltage.
A PWM signal voltage outputted from a comparator X4 is transmitted to a transistor Q1. Thus, transistors Q1, Q2 are turned on and a power source voltage is supplied to operational amplifiers X5, X6. A voltage proportional to an electric current flowing through the cold cathode discharge lamp is applied to both ends of a resistor R1. This voltage is rectified and smoothed by a diode D1 and a capacitor C9 and is applied to an inversion input terminal of the operational amplifier X6. The applied voltage is compared with a reference voltage Vref inputted to a non-inversion input terminal of the operational amplifier X6. An output voltage of the operational amplifier X6 is inputted to an inversion input terminal of the operational amplifier X5. On the other hand, a triangular wave voltage (normally ranging from 100 to 300 kHz) is inputted to a non-inversion input terminal of the operational amplifier X5, so that a PWM signal is outputted from an output terminal of the operational amplifier X5.
Transistors Q5 and Q6 are turned on during an H (high voltage) period of this PWM signal and a voltage is applied to a ROYER oscillating circuit so that an oscillating operation of the ROYER oscillating circuit is performed. Thus, an electric current flows through the cold cathode discharge lamp. Namely, as the electric current flowing through the cold cathode discharge lamp 11 is reduced and a voltage applied to the resistor R1 is reduced, the high (H) period of the PWM signal outputted from the operational amplifier X5 is lengthened (a duty ratio is increased) to elongate an oscillating period of the ROYER oscillating circuit 12. In contrast to this, conversely, when the electric current flowing through the cold cathode discharge lamp 11 is increased, the high (H) period is shortened (the duty ratio is reduced) to shorten the oscillating period of the ROYER oscillating circuit 12. Thus, the electric current flowing through the cold cathode discharge lamp becomes an approximately constant value even when the power voltage is changed.
However, there are the following defects when the DC/DC converter is used to reduce influences caused by the change in the power source voltage.
Power consumption of the DC/DC converter circuit is large and is increased by about 10% in comparison with the power consumption when the DC/DC converter circuit is not used. Further, cost of the lighting circuit is increased accompanying increase in the number of parts, and shortening of MTBF (mean time between failure), i.e., a reduction in reliability is caused. Furthermore, such a construction is contrary to a recent technical tendency of downsizing.